The results indicate distinct differences with maturation upon the neuronal-glial reaction of the hypoglossal nucleus to axonal injury in rats. Up to some 10 days postnatal, the neuronal membrane engulfs the presynaptic terminals in contact with its membrane and the proliferative glial phase is delayed to 1-3 weeks postoperative. In the mature neurone (21 plus days postnatal) the microglia proliferate earlier and lift the presynaptic terminals off the neuronal somata.